1. Field of the Invention
The present invention concerns a process for the conversion of caprolactam to nylon-6.
2. Related Art
Nylon-6 is produced commercially using caprolactam as the starting material. A commercial process typically involves heating caprolactam in a VK tube at atmospheric pressure for 12 to 24 hours. This process produces a nylon-6 product that contains 7 to 10% caprolactam in the product. Excess caprolactam is extracted using an 8 to 12 hour aqueous extraction process. The extracted product is then dried for 8 to 12 hours producing a dry product containing 0.2 to 0.3% caprolactam. While this process has met with commercial success, it has a long reaction residence time and a slow extraction/drying step.
U.S. Pat. No. 6,201,096 discloses a process for the production of a polyamide by reaction of an omega-aminonitrile with water in a vertical multistage reactor that is swept by steam. For example, this process can convert 6-aminocapronitrile to nylon-6.
The present invention is a process for making nylon-6, comprising:
(a) establishing a reactive distillation column having a top and a bottom, the top having a temperature of 150 to 200 degrees C. and the bottom having a temperature of 240 to 260 degrees C., the column having a plurality of spaced apart plates disposed therein, thereby defining a plurality of stages, each stage being defined by the column volume between adjacent plates, each stage being separately heated to a predetermined temperature;
(b) introducing caprolactam and water into a reaction zone having a temperature of 200 to 250 degrees C. and a pressure of 100 to 300 psia (0.69 to 2.07 MPa) to make a reaction product comprising caprolactam, linear aminocaproic acid and water;
(c) introducing the reaction product into the reactive distillation column at or near the top of the column while introducing steam at or near the bottom of the column; and
(d) recovering nylon 6 from the bottom of the column.
The process is expected to involve relatively short holdup times for the conversion of caprolactam to nylon-6. It is also expected to produce nylon-6 with relatively low amounts of free caprolactam and/or other low molecular weight compounds.